In commonly assigned application Ser. No. 07/822,016, now U.S. Pat. No. 5,293,125 (the entirety of which is incorporated herein by reference), there is disclosed a new and unique type of magneto-resistive tachometer. Briefly, in the aforementioned application, a magneto-resistive sensor arrangement is provided which utilizes a non-magnetically biased magneto-resistive sensor element; a magnetized drum; and associated integrated circuit based electronics. More specifically, the sensor arrangement includes an enclosure, a drum, a magneto-resistive probe, a hub, and an electronics module. The generally cylindrical enclosure is bolted onto the end plate of a motor casing or housing (e.g., a standard NEMA 4.5 inch and 8.5 inch C-Face end plate of an AC or DC electric motor). The enclosure provides a circular opening through which the motor shaft protrudes. In one exemplary embodiment, the hub is fastened to the end face of the motor shaft, and a magneto-resistive drum is fastened to the hub. The drum includes two magnetized tracks: an incremental magnetic ("INC") track and an index pulse ("Z") track. Such magnetized tracks are formed in a conventional manner by magnetizing the periphery of the drum. The "Z" track encodes one pattern (pulse) per revolution, while the "INC" track encodes (in the preferred embodiment) 480 patterns (pulses) per revolution.
The enclosure is formed with at least one, and preferably a pair of planar saddle surfaces, each having a rectangular opening therethrough. In one exemplary embodiment, a generally rectangular sensor housing or module is detachably fastened to the saddle surface and protrudes radially through the rectangular opening into the interior of the enclosure. The sensor housing supports a two track magneto-resistive probe at a free end thereof for sensing the "INC" track and the "Z" track, respectively. The preferred embodiment probe comprises a conventional magneto-resistive sensing element(s) and a miniature PC board providing signal processing circuitry.
The mounting arrangement between the enclosure and motor end plate, the enclosure and the sensor module, and the motor shaft and hub/drum provides a self-gapping probe-to-drum spacing of approximately 0.018 plus or minus 0.008 inches, and provides for self-alignment (axially and radially) between the probe and the drum. Thus, one significant innovation of the preferred embodiment design is that the magnetic drum and mounting hub, enclosure, and sensor module can be assembled onto a motor housing and put into operation without extensive alignment procedures. In other words, after the magnetic drum and hub have been fitted to the end face of the motor shaft and bolted in place, the enclosure is then bolted into place on the motor housing end plate. Finally, the sensor module is slipped into the radial opening in the enclosure and bolted into place on the saddle surface. The wires are connected and the tachometer is ready to operate, with no special adjustment of the location of the probe or sensor module required. With this configuration, the probe is removable and replaceable with another probe without any realignment of the assembly. This feature is advantageous for both assembly and service repair.
Such self-alignment and self-gapping is possible because electric motors are generally produced in accordance with NEMA standards which set the dimensions and tolerances of standard motor facings and shaft sizes. The preferred embodiment enclosure is machined to fit onto NEMA C type motor faces to a tight tolerance. The sensor module saddle mounting surface is also machined on the side of the enclosure to a tight tolerance with respect to the holding rim or lip of the enclosure which interfaces with the motor end plate. By holding these tight tolerances, as well as tight tolerances on the depth of the sensor from the mating surface of the probe to the surface of the sensor, the sensor can be located at a very precise distance from the center-line of the motor shaft.
In a like manner, the hub which is used to hold the magnetic drum onto the motor shaft is also machined to high tolerances. In one exemplary embodiment, the hub has a recess which fits over the end of the motor shaft. The recess is concentric to the center-line or axis of the shaft and to the outer diameter of the hub. Onto this the drum is centered with respect to the outer diameter of the hub held in place by screws or other suitable means (e.g., structural adhesive). The outer diameter of the drum is also machined to a tight tolerance. The depth of the hub recess determines the longitudinal location of the drum with respect to the sensor, and therefore, this depth is also tightly controlled.
In this configuration, the drum and sensor can be precisely located with respect to each other, across a radial gap. The tolerance of the gap between the sensor and drum edge is a function of the dimensional tolerances of the individual parts. According to the NEMA standards for one application, for example, the critical dimensions of the motor facing eccentricity is four thousands of an inch TIR (0.004"). The TIR of the shaft is three thousands of an inch (0.003"). The machining tolerances of the enclosure, probe and hub can be held to a few thousands of an inch or less. By using the tight machining tolerances as described above, the tolerance of the radial gap can be kept to plus-or-minus eight thousands of an inch (+/-0.008") or better.
In order to provide for the self-gapping arrangement as described in the aforementioned commonly owned application, and as explained above, the various components must be machined precisely to very close tolerances and is thus costly to manufacture.
The present invention seeks to alleviate this problem. Applicants have now devised a magneto-resistive tachometer construction which permits loose machining tolerances with final adjustment occurring during assembly on the production floor. The relaxed tolerances also allow for lower cost without sacrificing performance.
More specifically, the magneto-resistive tachometer in accordance with this invention utilizes a dual purpose cover. In normal use, the cover fits over the magnetic drum and electronic sensor to provide physical protection to the unit. In a shipping mode prior to use, however, the cover is reversed or put on backward, and in this orientation, serves as an alignment tool. In other words, with the cover installed in a reverse orientation, the radial gap between the sensor probe and the peripheral edge of the magnetic drum may be set during assembly. The cover is removed and reattached in its normal orientation only after the entire unit is secured to the motor casing, and this is done in such a way that the radial gap need not be reset.
In a presently preferred embodiment of the invention, the tachometer includes a magnetic sensor assembly, a magnetic drum assembly and a reversible cover. The magnetic sensor assembly includes a sensor module and a sensor module base unit which is adapted to receive the sensor module in a generally nested relationship. The sensor module includes probes which must be set at a predetermined radial distance from the peripheral edge of the magnetic drum, and therefore, means are provided which permit adjustment of the sensor module within the sensor module base unit. More specifically, the sensor module is mounted in the sensor module base unit for pivotal movement about a horizontal axis extending parallel to the centerline axis of the drum. The pivot pin is located at one end of the module, while a differentially threaded adjustment screw is located at tim other end of the sensor module, extending vertically through the sensor module and into the sensor module base unit. Rotation of the screw thus causes pivotal movement of the sensor module about the pivot pin such that, depending on the direction of rotation of the screw, the sensor module (and its magnetic probes) is moved toward or away the radially adjacent peripheral edge of the magnetic drum.
The magnetic drum assembly includes an annular drum having a peripheral edge with a pair of annular, magnetic "Z" and "INC" tracks thereon which must be placed in registry with the probes of the sensor module. The drum also includes an adaptor fastened thereto and provided with a central aperture adapted to be fitted over the output shaft of the motor. The magnetic drum assembly also includes one or more sleeve clamp which cooperates with the drum/adaptor to securely fasten the drum assembly to the output shaft of the motor as described in greater detail herein.
The reversible cover in accordance with an exemplary embodiment of the invention includes a main end wall having a large centrally located opening which is designed to seat an annular spacer member to which the magnetic drum assembly may be attached. Above the central aperture, a planar surface of the main end wall is provided with bolt apertures to facilitate mounting of the sensor module base unit (and the sensor module) thereto, directly above and in required radial alignment with the magnetic drum assembly.
The cover also includes side and bottom walls which serve to at least partially enclose the magnetic drum assembly when in its normal, in-use orientation secured to the motor end plate or casing.
During the initial assembly of the unit at the factory, the sensor module assembly (the sensor module and the sensor module base unit) is attached to one portion of the cover, while the magnetic drum is secured to another portion of the cover directly beneath the sensor module assembly. The radial gap between the peripheral edge (incorporating the "Z" and "INC" tracks) of the magnetic drum and magneto-resistive sensor module probes is then preset to the required dimension using the differential screw. The entire unit is then shipped to the customer. After the customer receives the unit, the latter is fixed to the motor casing, with the magnetic drum assembly slipped over the motor shaft until the sensor module base unit abuts the motor casing or end plate. The sensor module base unit is then bolted to the end plate or casing, and the magnetic drum assembly is then secured to the shaft. Since the cover maintains the relative position between the sensor module base unit and the magnetic drum, the customer is assured that the critical gap dimension is maintained. With the sensor module base unit and magnetic drum assembly now also fixed to the motor casing and output shaft, respectively, the cover may be removed without changing the gap. The cover is then reversed and refastened to the opposite sides of the magnetic sensor base unit and magnetic drum in its normal, in-use orientation to provide physical protection to the unit.
In the event, however, the radial gap is altered, the differential screw may again be employed to easily re-set the gap to the required dimension.
In accordance with an exemplary embodiment of the invention, therefore, a tachometer assembly is provided which comprises a sensor module assembly; a magnetic drum assembly; and cover supporting the sensor module assembly and the magnetic drum in predetermined radial alignment with each other in a first orientation of the cover; the cover adapted to partially enclose and protect the sensor module assembly and the magnetic drum assembly in a second orientation reversed relative to the first orientation.
In another aspect, the invention provides a tachometer assembly comprising a magnetic drum having a peripheral edge surface, the drum adapted for attachment to an output shaft of a rotary machine; a sensor module; a sensor module base unit supporting the sensor module, the sensor module base unit adapted for attachment to a casing of the rotary machine; and a cover supporting, in a first, non-use orientation, the magnetic drum assembly, the sensor module and the sensor module base unit in a spatial relationship such that a predetermined radial gap between the sensor module and the peripheral edge surface of the magnetic drum assembly is established; the magnetic drum assembly having means for attaching the magnetic drum assembly to the output shaft, and the sensor module base unit having means for attaching the sensor module base unit to an end plate of the rotary machine with the cover in the first non-use position, the cover ada for removal, reversal and remounting to the rotary machine casing in a second, in-use orientation which is reversed relative to the first, non-use orientation, and without disturbing the predetermined radial gap.
In still another aspect, the invention provides a tachometer assembly which comprises a cover unit having an end wall with front and back surfaces, a pair of side walls extending from the front surface of the end wall, and a bottom wall extending from the front surface of the end wall between the side walls, the end wall having a centrally located aperture therein; a magnetic drum assembly having a circumferential peripheral edge surface provided with at least one magnetic pattern thereon; a sensor module assembly including at least one sensing probe; wherein the magnetic drum assembly is mounted on the back surface of the end wall substantially concentrically with the centrally located apertures, and the sensor module assembly is mounted on the back surface of the end wall in a first shipping orientation to maintain at least one sensing probe a predetermined radial distance from the magnetic pattern; and wherein the cover unit is adapted to be reversed to a use orientation such that the sensor module assembly and the magnetic drum assembly will engage the front surface of the cover unit and the side walls and the bottom wall will at least partially enclose the sensor module assembly and the magnetic drum assembly.
Finally, the invention provides a method of assembling and aligning components of a tachometer relative to each other and to a shaft of a rotary machine, the components including a magnetic drum, a periphery of which is provided with at least one magnetic pattern; a sensor module assembly including a sensor module having a probe, and a sensor module base unit adjustably supporting the sensor module; and a cover unit, the method comprising the steps of:
a) mounting the magnetic drum and the sensor module assembly on one side of the cover;
b) adjusting said sensor module and probe relative to said sensor module base unit and said magnetic drum to obtain a predetermined radial gap between the probe and the magnetic pattern of the magnetic drum;
c) securing the magnetic drum on the rotary machine shaft;
d) securing the sensor module assembly to a casing of the rotary machine through which the shaft projects;
e) removing the cover unit, leaving the magnetic drum on the shaft and the sensor module assembly on the casing of the rotary machine;
f) reversing the cover unit; and
g) reattaching it to the casing.
The invention as described herein provides unique advantages in terms of radial gap setting, cost, simplicity and ease of manufacture and use.
Additional objects and advantages of the invention will become apparent from the detailed description which follows.